When a fuel injector injects fuel into a combustion chamber of an internal combustion engine, there is a time delay from when a fuel-injection command signal is transmitted until when the fuel is actually injected. Each fuel injector has an individual variation in a correlation between an output period of the fuel-injection command signal and the fuel injection quantity. The time delay and the fuel injection correlation are previously obtained by experiments and are stored in a memory as a fuel-injection-characteristic value. After the fuel injector is shipped, based on the stored fuel-injection-characteristic value, the fuel-injection command signal is established.
JP-2009-74535A (US-2009-0056678A1) and JP-2009-57926A (US-2009-0056676A1) show that a fuel pressure sensor is provided to a fuel injector in order to detect a variation in fuel pressure (fuel pressure waveform). Based on this variation in fuel pressure, a variation in fuel-injection-rate (fuel injection condition) is analyzed. For example, when a fuel injection is started, the fuel pressure waveform starts to descend due to the fuel injection. Thus, based on a time when the fuel pressure waveform starts to descend, the fuel-injection-start time can be computed (analyzed).
According to above, even after the fuel injector is shipped, the actual fuel injection condition can be analyzed so that the fuel-injection-characteristic value can be detected. Even if the fuel-injection-characteristic value is varied due to an aging deterioration, the fuel-injection-characteristic value can be learned so that the fuel injection condition can be controlled with high accuracy.
Meanwhile, the fuel-injection-characteristic value depends on a fuel temperature. If the fuel-injection-characteristic value is learned without respect to the fuel temperature and the fuel-injection command signal is established, the fuel injection condition can not be accurately controlled. The present inventors have found out such problems.